The Electron Electric Dipole Moment: How the Shape of the Electron’s Field could Hint at New Particles

The matter in our universe is made of atoms; atoms are made of protons, neutrons, and electrons; and protons and neutrons are made of quarks. In our current theory of particle physics, electrons and quarks are elementary particles–they are the foundation of everything else. The full catalog of all known elementary particles and their interactions is called the Standard Model (SM). The SM has been extremely successful at describing the behaviors of known particles and predicting the existence of new ones, such as the Higgs boson, which was recently discovered at CERN in 2012. However, several observations suggest there are still unknown particles and forces in our universe, which means the SM is incomplete.

Physicists look for new particles in several ways. One way is to indirectly observe the effects of their interactions. For example, you can observe an electromagnetic field by using a magnet to move a paperclip. Consider an electron: a round particle with electric charge. The electromagnetic field of an isolated electron is spherical. This field is actually built out of other particles called photons, which are being exchanged between the electron and other charged particles. If there were other charged particles present, then we would see evidence of other particle interactions: the field would no longer be spherical. Thus by measuring the ‘shape’ of the electron’s field, scientists can determine if new interactions are present. This would indicate the existence of unknown particles.

The ACME collaboration, led by John Doyle at Harvard, Gerald Gabrielse at Northwestern, and David DeMille at Yale, recently announced a measurement of unprecedented precision of the electron electric dipole moment (eEDM). Dipoles occur in electromagnetism in macroscopic systems, like a bar magnet with a north and south pole, and indicate a separation in charge. Therefore, we do not expect this behavior in a fundamental point particle. If the electron had a separation of charge (aka a ‘nonzero eEDM’), the field would no longer look spherical and would instead look ‘flattened.’ On the particle level, the change in field shape is due to interactions with new, heavy, charged particles that we have not yet been able to detect. The ACME experiment was sensitive enough that if the electron were the size of the Earth, it could detect a field deformation a million times smaller than the width of a hair. To achieve this incredible degree of sensitivity, the experiment used new techniques with lasers and magnetic fields to orient charged particles. A misalignment of the particles would indicate a nonspherical electron field, and therefore a nonzero eEDM. At this level of precision, no such defect was observed, thus placing a tight upper bound on the eEDM value.

Had the ACME collaboration measured a nonzero eEDM, this would imply the existence of a new particle. Since all scientific measurements have some uncertainty, researchers can never claim the eEDM is exactly zero, either. However, as the uncertainty on the eEDM measurement shrinks with high-resolution experiments, it is less likely that undetected new particle interactions are occurring and we are just missing them..

Several prominent theories in particle physics, such as supersymmetry (SUSY), predict a nonzero eEDM. SUSY is one of the most popular theories that expands upon the SM. SUSY explains why particles have the mass they do (instead of being much heavier), a crucial piece of the SM that physicists do not yet understand. If SUSY is the correct theory of the universe, we will eventually measure a nonzero eEDM, since it would have to exist. Since ACME did not measure an eEDM with their current sensitivity, researchers still do not know if SUSY is real or if there are new heavy particles. Instead, we have to wait for the next, more precise measurement.